The hydrogen-oxygen fuel cell receives attention as a power generating system having little adverse effect on the global environment because in principle, its reaction product is water only. Among such fuel cells, polymer electrolyte fuel cells are greatly expected to be used practically, since their power density has improved with the remarkable research progress in recent years.
A polymer electrolyte fuel cell has a membrane-electrode assembly which comprises gas diffusion electrodes each having a catalyst layer containing a catalyst as the anode and the cathode and an electrolyte membrane bonded to (or in contact with) the cathode and the anode flanking on both sides of the electrolyte membrane. When a fuel gas and an oxidant gas containing oxygen are supplied to the anode and the cathode of a membrane-electrode assembly, respectively, electrochemical reactions proceed inside to generate electric energy.
To improve the performance of such polymer electrolyte fuel cells, various methods for producing a membrane-electrode assembly have been studied so far, and, for example, the following methods are known.
(1) A method comprising depositing a catalyst directly on an electrolyte membrane. (2) A method comprising preparing gas diffusion electrode sheets having catalytic power and bonding the electrode sheets to an electrolyte membrane. (3) A method for producing a membrane-electrode assembly comprising preparing two electrolyte membranes (half-cells) each having a catalyst layer formed thereon and bonding the electrolyte membranes under pressure with the electrolyte membrane sides face-to-face.
Especially, the method (2) is widely used because of its advantage that small amounts of the catalysts can be used effectively. As a specific example of the method (2), a method comprising forming catalyst layers on additional base films, and transferring the catalyst layers onto an electrolyte membrane by laminating the electrolyte membrane to the catalyst layers by hot pressing (hereinafter referred to as “the transfer method”) was proposed.
However, when an increase in performance of a membrane-electrode assembly is aimed by the above transfer method using an electrolyte membrane thinner than 30 μm, because such a membrane has low mechanical and tensile strengths and therefore has problematic workability and handleability, the resulting membrane-electrode assembly having such an insufficiently durable electrolyte membrane has a problem that its properties remarkably deteriorate during long operation.
Particularly, when catalyst layers are formed by coating the gas diffusion layers with a coating solution, because the gas diffusion layers are usually made of porous carbon paper or carbon felt, some carbon fibers protruding from the surfaces of the gas diffusion layers bite into the catalyst layers, and further into the electrolyte membrane at the time of bonding the electrodes and the electrolyte membrane by hot pressing, and therefore, gas leakage tends to occur. As a result, the open circuit voltage of the membrane-electrode assembly tends to drop, and the anode and the cathode tend to short-circuit. Accordingly, this method hardly provides a membrane-electrode assembly using a thin electrolyte membrane having a thickness of at most 30 μm and has limitation on how much output characteristics can be improved while maintaining good durability.
Further, attempts to increase the sulfonic acid group concentration in an electrolyte membrane have been made with a view to improving the performance of a membrane-electrode assembly by reducing the resistance of the electrolyte membrane. However, a drastic increase in the sulfonic acid group concentration in the membrane tends to deteriorate the mechanical and tensile strength of the membrane and tends to cause dimensional change due to atmospheric moisture when the membrane is handled. Further, a membrane-electrode assembly prepared by using such an electrolyte membrane vulnerable to creeping and insufficient in durability, remarkably deteriorates in terms of characteristics during long operation.
Further, an electrolyte membrane tends to cause various troubles upon hydration by swelling up in the longitudinal direction. For example, if an electrolyte fuel cell provided with a membrane-electrode assembly is operated, the membrane swells up to a larger size with water produced by the reaction or water vapor supplied together with the fuel gas. Because the membrane is usually bonded to electrodes, the electrodes also undergo dimensional change after the membrane. The assembly is usually bound to e.g. a separator having grooves formed as gas channels, and therefore, the membrane offsets its dimensional increase by wrinkling. The wrinkles can interrupt the gas flow by blocking the groove in the separator.
As a solution to the above-mentioned problems, a polytetrafluoroethylene (hereinafter referred to as PTFE) porous membrane impregnated with a fluorinated ion exchange polymer having sulfonic acid groups was proposed (e.g. JP-B-5-75835, claims 1 and 2). However, being a relatively soft material, a porous PTFE does not have sufficient reinforcing effect and falls short of a solution to the above-mentioned problems. Further, a porous polyolefin filled with an ion exchange resin was proposed (JP-B-7-68377). However, there is a problem in long-term stability with a hydrocarbon type resin because the chemical resistance is required for ion exchange membrane under operation of the fuel cell.